A driving motor of which the speed can be continuously varied by means of a control member, in particular a diesel engine, produces a torque that is nearly independent of its speed. Vehicles and driven machines however require very variable driving forces to overcome load or running resistances such as slopes, to produce thrust and/or tractive forces or to accelerate. Hence it should be possible to convert the power designed into the motor as completely as possible into working power in the drive system. This requirement is met by a converter connected between the driving motor and the transmission with a control dependent on the load (pressure) and input speed (driving speed) which decreases the transmission ratio of the converter as the speed to be delivered by the driving motor increases, and increases it as the load (running pressure) to be applied by the driving motor increases. Hence the purpose of the transmission (often a 2 or 3 gear transmission) is to increase the available range of transmission ratios of the drive system.
During the working phases of, e.g. a wheel loader, it always runs in first gear and varies its running speed by means of the variable diesel engine speed and by means of the converter. By using a multiple-ratio change-under-load transmission it is possible to change the mechanical transmission ratio even while running and even when under load. If in this case no measures are taken to adapt the output speed of the converter and the input speed of the mechanical transmission to one another, then owing to the stiffness of the overall drive complex a strong shifting jerk occurs which, depending on the actual running condition of the wheel loader, can interfere with the running behavior and the running stability to a greater or lesser extent.
With a drive system of the kind mentioned in the introduction, described and illustrated in DE-OS No. 34 33 494, the converter and its control means are used to synchronize the output speed of the converter with the input speed of the transmission that is to be expected after the shifting operation. Here the control means must carry out a completely different control response than is specified for it by the speed control. In this known arrangement an extremely complex control means is necessary, which not only increases manufacturing costs but also leads to increased wear and increased use of fuel. While synchronization can prevent jerks when shifting, in this known arrangement there are adaptation difficulties in the transition region between the synchronization control and the speed control.